A Wi-Fi network may be formed by one or more access points (APs) that provide a wireless communication channel or link with a number of client devices or stations (STAs). Each AP, which may correspond to a Basic Service Set (BSS), periodically broadcasts beacon frames to enable any STAs within wireless range of the AP to establish and/or maintain a communication link with the Wi-Fi network. The beacon frames, which may include a traffic indication map (TIM) indicating whether the AP has queued downlink data for the STAs, are typically broadcast according to a target beacon transmission time (TBTT) schedule. The broadcasted beacon frame may also include the AP's timing synchronization function (TSF) value. The STAs may synchronize their own local TSF values with the broadcasted TSF value, for example, so that all the STAs are synchronized with each other and the AP.
In a typical WLAN, only one device may use the wireless medium at any given time. To arbitrate access to the shared wireless medium, the IEEE 802.11 standards define a distributed coordination function (DCF) that instructs individual STAs (and APs) to “listen” to the medium to determine when the medium is idle (e.g., using a “carrier sense” technique). For example, only when a STA detects that the medium has been continuously idle for a DCF lnterframe Space (DIFS) duration may the STA attempt to transmit data on the medium. To prevent multiple devices from accessing the medium at the same time, each device may select a random “back-off” number or period. At the end of the DIFS duration, a contention period begins during which each device waits for a period of time determined by its back-off number (e.g., its back-off period) before it attempts to transmit data on the medium. The device that selects the lowest back-off number will have the shortest back-off period, and therefore “wins” the medium access contention operation. The winning device may be granted access to the shared wireless medium for a period of time commonly referred to as a transmit opportunity (TXOP), during which the winning device may transmit data over the shared wireless medium.
If multiple devices select the same back-off value and then attempt to transmit data at the same time, a collision would occur and thus the devices contend for medium access again using an exponential back-off procedure. More specifically, after a collision is detected, each device selects another random number, and then doubles the selected random number to generate the device's next back-off value. Although doubling the size of the back-off values for successive contention periods may reduce the chance of collisions (e.g., as compared with earlier contention periods that resulted in collisions), transmission latencies are undesirably increased. These transmission latencies are exacerbated as the number of devices in the WLAN system increases. For example, for a typical WLAN system, the average transmission latency may be 4 ms or greater.
The transmission latencies for WLAN systems may result in such WLAN systems being unsuitable for applications requiring smaller transmission latencies. For example, many robotics, industrial controls systems, and real-time computing applications require transmission latencies on the order of 1 ms, which is much less than the transmission latency of a typical WLAN system.
Thus, there is a need to reduce transmission latencies for WLAN systems without adversely affecting performance.